Piezoelectric elements are used, for example, as piezo injectors in injection systems for motor vehicles. They are charged to a specific voltage that is proportional to a lift of the piezoelectric element. Transmitted with the aid of a hydraulic coupler, this lift actuates an injection valve. To ensure the full functionality of such a piezo injector, the control voltage for every operating point, for example in the case of an injection system for a motor vehicle for every pressure in the common-rail system, for every temperature and the like, must be selected in such a way that the control valve opens and remains in the open position for the duration of the injection cycle.
In the method described in German Patent Application No. DE 100 32 022, for example, the control voltage, which is individual in every piezoelectric element (individual control voltage), is ascertained by indirectly measuring the pressure in the hydraulic coupler prior to the injection process. From the pressure induced by the hydraulic coupler in the piezo actuator, a corresponding piezo voltage in the actuator is inferred.
Furthermore, methods are also known in which the voltage is varied in certain regions and at the same time the injection quantity is measured. The operativeness of the piezoelectric element is inferred from the measured injection quantity.
A method and a circuit for the time-dependent measurement of the voltage along a piezoelectric element emerges for example from European Patent Application No. EP 1 138 902.
When manufacturing piezo elements that are used in common-rail diesel injection systems, it is necessary to adhere to very narrow tolerances in the manufacture of such piezo common-rail injectors so as to yield the highest possible output in the series. Very important for this purpose is the interaction between the piezo actuator and the hydraulic system, for example, in the form of the hydraulic coupler. Since in injectors of this kind no sensors can be used for reasons of costs, only little information is available regarding the dynamic behavior of piezo injectors.
Until now, changes of piezo actuators as a result of aging, for example, an aging of the piezo ceramics or the breaking of valve seats, can only be compensated at least partially by previously defined characteristics maps, which are stored for example in a control unit of an internal combustion engine.
An objective of the present invention is to ascertain, in a manner that is technically as simple as possible and as cost-effective as possible, the individual control voltage of piezo elements, particularly piezo injectors used particularly in diesel common-rail injection systems. In so doing, the dynamics of an individual control voltage, which changes over the service life of a piezo injector, is to be taken into account as well.